Domestic gas product from an lng facility

ABSTRACT

An LNG facility capable of producing a domestic gas product from an intermediate stream in the LNG facility. Withdrawing the domestic gas product from a location within the LNG facility can minimize operational disturbances typically caused by extracting a domestic gas product stream upstream of the liquefaction portion of the LNG facility. In addition, withdrawing the domestic gas product from this location can provide increased control of light contaminants (e.g., nitrogen) in open-loop refrigeration cycles and can ultimately result in incremental LNG and/or NGL production.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to methods and apparatuses for liquefying naturalgas. In another aspect, the invention concerns an LNG facility capableof simultaneously producing liquefied natural gas (LNG) and a domesticgas product.

2. Description of the Prior Art

Cryogenic liquefaction is commonly used to convert natural gas into amore convenient form for transportation and/or storage. Becauseliquefying natural gas greatly reduces its specific volume, largequantities of natural gas can be economically transported and/or storedin liquefied form.

Transporting natural gas in its liquefied form can effectively link anatural gas source with a distant market when the source and market arenot connected by a pipeline. This situation commonly arises when thesource of natural gas and the market for the natural gas are separatedby large bodies of water. In such cases, LNG can be transported from thesource to the market using specially designed ocean-going LNG tankers.

Storing natural gas in its liquefied form can help balance out periodicfluctuations in natural gas supply and demand. In particular, LNG can be“stockpiled” for use when natural gas demand is low and/or supply ishigh. As a result, future demand peaks can be met with LNG from storage,which can be vaporized as demand requires.

Several methods exist for liquefying natural gas. Some methods produce apressurized LNG (PLNG) product that is useful, but requires expensivepressure-containing vessels for storage and transportation. Othermethods produce an LNG product having a pressure at or near atmosphericpressure. In general, these non-pressurized LNG production methodsinvolve cooling a natural gas stream via indirect heat exchange with oneor more refrigerants and then expanding the cooled natural gas stream tonear atmospheric pressure. In addition, most LNG facilities employ oneor more systems to remove contaminants (e.g., water, acid gases,nitrogen, and ethane and heavier components) from the natural gas streamat different points during the liquefaction process.

In addition to LNG, some LNG facilities also produce a domestic gasproduct. As used herein, the term “domestic gas product” refers to anygaseous, predominantly methane stream originating from an LNG facilitythat is routed to a location external to the LNG facility for saleand/or use. Typically, domestic gas products from LNG facilities aretransported via pipeline to the local natural gas market for subsequentsale. The domestic gas product from most LNG facilities originates as aslip stream of the natural gas feed entering the liquefaction portion ofthe LNG facility. In order to ensure the domestic gas product meetscertain pipeline specifications (e.g., hydrocarbon dew point), thewithdrawn natural gas stream is often subjected to further processing(e.g., distillation) in order to produce a compliant domestic gasproduct. Often, the remaining portion of the domestic gas stream isrecombined with the natural gas feed stream entering the LNG facility, apractice which can cause in drastic changes in the composition of thenatural gas feed. These drastic changes can adversely affect theoperation of the LNG facility and can ultimately result in off-spec LNGproduct and/or reduced LNG production.

Thus, a need exists for an LNG facility that is capable of efficientlyand consistently producing on-spec LNG and a pipeline-compliant domesticgas product without requiring additional process equipment in order tomaximize facility production while minimizing capital and operatingcosts.

SUMMARY OF THE INVENTION

In one embodiment of the present invention, there is provided a processfor liquefying a natural gas stream in an LNG facility, the processcomprising: (a) cooling at least a portion of the natural gas stream ina first refrigeration cycle via indirect heat exchange with apredominantly methane refrigerant; (b) flashing at least a portion ofthe cooled natural gas stream to thereby provide a predominantly liquidproduct stream and a predominantly vapor refrigerant stream; (c)compressing at least a portion of the predominantly vapor refrigerantstream to thereby provide a compressed refrigerant stream; and (d)separating at least a portion of the compressed refrigerant stream intoa domestic gas fraction and a compressed refrigerant fraction.

In another embodiment of the present invention, there is provided aprocess for liquefying a natural gas stream in an LNG facility, theprocess comprising: (a) cooling at least a portion of the natural gasstream in an upstream refrigeration cycle of the LNG facility viaindirect heat exchange with an upstream refrigerant to thereby provide acooled natural gas stream; (b) further cooling at least a portion of thecooled natural gas stream via indirect heat exchange with apredominantly methane refrigerant stream in a methane refrigerationcycle to thereby produce a further cooled natural gas stream and awarmed refrigerant stream; (c) separating at least a portion of thewarmed refrigerant stream into a domestic gas fraction and a refrigerantfraction; and (d) cooling at least a portion of the refrigerant fractionin the upstream refrigeration cycle via indirect heat exchange with theupstream refrigerant.

In yet another embodiment of the present invention, there is provided aprocess for liquefying a natural gas stream in an LNG facility, theprocess comprising: (a) cooling the natural gas stream in a firstrefrigeration cycle via indirect heat exchange with a first refrigerantto thereby produce a cooled predominantly methane stream; (b) separatingat least a portion of the cooled predominantly methane stream in adistillation column to thereby produce a heavies-rich stream and aheavies-depleted stream; (c) subjecting at least a portion of theheavies-depleted stream to expansion cooling to thereby produce LNGhaving a pressure in the range of from about 0 to about 40 psia; and (d)prior to at least a portion of the expansion cooling of step (c),withdrawing a domestic gas fraction from the heavies-depleted stream.

In a still further embodiment of the present invention, there isprovided an LNG facility for liquefying a natural gas stream. The LNGfacility comprises an open-loop refrigeration cycle operable to cool atleast a portion of the natural gas stream via indirect heat exchangewith a first refrigerant. The open-loop refrigeration cycle comprises afirst heat exchanger defining a first cooling pass and a firstrefrigerant pass. The first heat exchanger is operable to cool at leasta portion of the natural gas stream in the first cooling pass viaindirect heat exchange with the first refrigerant in the firstrefrigerant pass. The open-loop refrigeration cycle also comprises anexpander defining an expander inlet and an expander outlet. The expanderinlet is in fluid communication with the first cooling pass. Theopen-loop refrigeration cycle further comprises a vapor-liquid separatordefining a separator inlet, a lower liquid outlet, and an upper vaporoutlet. The separator inlet is coupled in fluid flow communication withthe expander outlet and the upper vapor outlet is coupled in fluid flowcommunication with the first refrigerant pass. The open-looprefrigeration cycle also comprises a first refrigerant compressordefining an inlet port and an outlet port. The inlet port is coupled influid flow communication with the first refrigerant pass. The open-looprefrigeration cycle additionally comprises a compressed refrigerantconduit for routing fluid flow from the outlet port of the compressor toa location within the LNG facility and a domestic gas conduit forrouting fluid flow from the outlet port of the compressor and/or thecompressed refrigerant conduit to a location outside the LNG facility.

BRIEF DESCRIPTION OF THE DRAWINGS

Certain embodiments of the present invention are described in detailbelow with reference to the enclosed figures, wherein:

FIG. 1 is a simplified overview of a cascade-type LNG facility inconfigured in accordance with one embodiment of the present invention;and

FIG. 2 is a schematic diagram a cascade-type LNG facility configured inaccordance with one embodiment of present invention.

DETAILED DESCRIPTION

The present invention can be implemented in a facility used to coolnatural gas to its liquefaction temperature to thereby produce liquefiednatural gas (LNG). The LNG facility generally employs one or morerefrigerants to extract heat from the natural gas and then reject theheat to the environment. Numerous configurations of LNG systems exist,and the present invention may be implemented many different types of LNGsystems.

In one embodiment, the present invention can be implemented in a mixedrefrigerant LNG system. Examples of mixed refrigerant processes caninclude, but are not limited to, a single refrigeration system using amixed refrigerant, a propane pre-cooled mixed refrigerant system, and adual mixed refrigerant system.

In another embodiment, the present invention is implemented in a cascadeLNG system employing a cascade-type refrigeration process using one ormore pure component refrigerants. The refrigerants utilized incascade-type refrigeration processes can have successively lower boilingpoints in order to maximize heat removal from the natural gas streambeing liquefied. Additionally, cascade-type refrigeration processes caninclude some level of heat integration. For example, a cascade-typerefrigeration process can cool one or more refrigerants having a highervolatility via indirect heat exchange with one or more refrigerantshaving a lower volatility. In addition to cooling the natural gas streamvia indirect heat exchange with one or more refrigerants, cascade andmixed-refrigerant LNG systems can employ one or more expansion coolingstages to simultaneously cool the LNG while reducing its pressure tonear atmospheric pressure.

FIG. 1 illustrates one embodiment of a simplified LNG facility capableof simultaneously producing LNG and a domestic gas product. Thecascade-type LNG facility of FIG. 1 generally comprises a cascadecooling section 10, a heavies removal zone 11, and an expansion coolingsection 12. Cascade cooling section 10 is depicted as comprising a firstmechanical refrigeration cycle 13, a second mechanical refrigerationcycle 14, and a third mechanical refrigeration cycle 15. In general,first, second, and third refrigeration cycles 13, 14, 15 can beclosed-loop refrigeration cycles, open-loop refrigeration cycles, or anycombination thereof. In one embodiment of the present invention, firstand second refrigeration cycles 13 and 14 can be closed-loop cycles, andthird refrigeration cycle 15 can be an open-loop cycle that utilizes arefrigerant comprising at least a portion of the natural gas feed streamundergoing liquefaction.

In accordance with one embodiment of the present invention, first,second, and third refrigeration cycles 13, 14, 15 can employ respectivefirst, second, and third refrigerants having successively lower boilingpoints. For example, the first, second, and third refrigerants can havemid-range boiling points at standard pressure (i.e., mid-range standardboiling points) within about 20° F., within about 10° F., or within 5°F. of the standard boiling points of propane, ethylene, and methane,respectively. In one embodiment, the first refrigerant can comprise atleast about 75 mole percent, at least about 90 mole percent, at least 95mole percent, or can consist essentially of propane, propylene, ormixtures thereof. The second refrigerant can comprise at least about 75mole percent, at least about 90 mole percent, at least 95 mole percent,or can consist essentially of ethane, ethylene, or mixtures thereof. Thethird refrigerant can comprise at least about 75 mole percent, at leastabout 90 mole percent, at least 95 mole percent, or can consistessentially of methane.

As shown in FIG. 1, first refrigeration cycle 13 can comprise a firstrefrigerant compressor 16, a first cooler 17, and a first refrigerantchiller 18. First refrigerant compressor 16 can discharge a stream ofcompressed first refrigerant, which can subsequently be cooled and atleast partially liquefied in cooler 17. The resulting refrigerant streamcan then enter first refrigerant chiller 18, wherein at least a portionof the refrigerant stream can cool the incoming natural gas stream inconduit 100 via indirect heat exchange with the vaporizing firstrefrigerant. The gaseous refrigerant can exit first refrigerant chiller18 and can then be routed to an inlet port of first refrigerantcompressor 16 to be recirculated as previously described.

First refrigerant chiller 18 can comprise one or more cooling stagesoperable to reduce the temperature of the incoming natural gas stream inconduit 100 by about 40 to about 210° F., about 50 to about 190° F., or75 to 150° F. Typically, the natural gas entering first refrigerantchiller 24 via conduit 100 can have a temperature in the range of fromabout 0 to about 200° F., about 20 to about 180° F., or 50 to 165° F.,while the temperature of the cooled natural gas stream exiting firstrefrigerant chiller 18 can be in the range of from about −65 to about 0°F., about −50 to about −10° F., or −35 to −15° F. In general, thepressure of the natural gas stream in conduit 100 can be in the range offrom about 100 to about 3,000 pounds per square inch absolute (psia),about 250 to about 1,000 psia, or 400 to 800 psia. Because the pressuredrop across first refrigerant chiller 18 can be less than about 100 psi,less than about 50 psi, or less than 25 psi, the cooled natural gasstream in conduit 101 can have substantially the same pressure as thenatural gas stream in conduit 100.

As illustrated in FIG. 1, the cooled natural gas stream (also referredto herein as the “cooled predominantly methane stream”) exiting firstrefrigeration cycle 13 can then enter second refrigeration cycle 14,which can comprise a second refrigerant compressor 19, a second cooler20, and a second refrigerant chiller 21. Compressed refrigerant can bedischarged from second refrigerant compressor 19 and can subsequently becooled and at least partially liquefied in cooler 20 prior to enteringsecond refrigerant chiller 21. Second refrigerant chiller 21 can employa plurality of cooling stages to progressively reduce the temperature ofthe predominantly methane stream in conduit 101 by about 50 to about180° F., about 65 to about 150° F., or 95 to 125° F. via indirect heatexchange with the vaporizing second refrigerant. As shown in FIG. 1, thevaporized second refrigerant can then be returned to an inlet port ofsecond refrigerant compressor 19 prior to being recirculated in secondrefrigeration cycle 14, as previously described.

The natural gas feed stream in conduit 100 will usually contain ethaneand heavier components (C₂+), which can result in the formation of a C₂+rich liquid phase in one or more of the cooling stages of secondrefrigeration cycle 14. In order to remove the undesired heaviesmaterial from the predominantly methane stream prior to completeliquefaction, at least a portion of the natural gas stream passingthrough second refrigerant chiller 21 can be withdrawn via conduit 102and processed in heavies removal zone 11, as shown in FIG. 1. Thenatural gas stream in conduit 102 can have a temperature in the range offrom about −160 to about −50° F., about −140 to about −65° F., or to−85° F. and a pressure that is within about 5 percent, about 10 percent,or 15 percent of the pressure of the natural gas feed stream in conduit100.

Heavies removal zone 11 can comprise one or more gas-liquid separatorsoperable to remove at least a portion of the heavy hydrocarbon materialfrom the predominantly methane stream. Typically, heavies removal zone11 can be operated to remove benzene and other high molecular weightaromatic components, which will freeze in subsequent liquefaction stepsand plug downstream process equipment. In addition, heavies removal zone11 can be operated to recover the heavy hydrocarbons as a natural gasliquids (NGL) product stream. Examples of typical hydrocarbon componentscomprising NGL streams can include ethane, propane, butane isomers,pentane isomers, and hexane and heavier components (i.e., C₆+). Theextent of NGL recovery from the predominantly methane stream canultimately impact one or more final characteristics of the LNG product,such as, for example, Wobbe index, BTU content, higher heating value(HHV), ethane content, and the like. In one embodiment, the NGL productstream exiting heavies removal zone 11 can be subjected to furtherfractionation in order to produce one or more pure component streams.Often, NGL product streams and/or their constituents can be used asgasoline blendstock.

The predominantly methane stream exiting heavies removal zone 11 viaconduit 103 can comprise less than about 1 weight percent, less thanabout 0.5 weight percent, less than about 0.1 weight percent, or lessthan 0.01 weight percent of C₆+ material, based on the total weight ofthe stream. Typically, the predominantly methane stream in conduit 103can have a temperature in the range of from about −140 to about −50° F.,about −125 to about −60° F., or −110 to −75° F. and a pressure in therange of from about 200 to about 1,200 psia, about 350 to about 850psia, or 500 to 700 psia. As shown in FIG. 1, the stream exiting heaviesremoval zone 12 via conduit 103 can subsequently be routed back tosecond refrigeration cycle 14, wherein the stream can be further cooledvia second refrigerant chiller 21. In one embodiment, the stream exitingsecond refrigerant chiller 21 via conduit 104 can be completelyliquefied and can have a temperature in the range of from about −205 toabout −70° F., about −175 to about −95° F., or −140 to −125° F.Generally, the stream in conduit 104 can be at approximately the samepressure the natural gas stream entering the LNG facility in conduit100.

As illustrated in FIG. 1, the pressurized LNG-bearing stream in conduit104 enters third refrigeration cycle 15, which is depicted as generallycomprising a third refrigerant compressor 22, a cooler 23, and a thirdrefrigerant chiller 24. Compressed refrigerant discharged from thirdrefrigerant compressor 22 enters cooler 23, wherein the refrigerantstream is cooled and at least partially liquefied prior to enteringthird refrigerant chiller 24. Third refrigerant chiller 24 can compriseone or more cooling stages operable to subcool the pressurizedpredominantly methane stream via indirect heat exchange with thevaporizing refrigerant. In one embodiment, the temperature of thepressurized LNG-bearing stream can be reduced by about 2 to about 60°F., about 5 to about 50° F., or 10 to 40° F. in third refrigerantchiller 24. In general, the temperature of the pressurized LNG-bearingstream exiting third refrigerant chiller 24 via conduit 105 can be inthe range of from about −275 to about −75° F., about −225 to about −100°F., or −200 to −125° F.

As shown in FIG. 1, the pressurized LNG-bearing stream in conduit 105can be then routed to expansion cooling section 12, wherein the streamis sub-cooled via sequential pressure reduction to near atmosphericpressure by passage through one or more expansion stages. In oneembodiment, each expansion stage can reduce the temperature of theLNG-bearing stream by about 10 to about 60° F., about 15 to about 50°F., or 20 to 40° F. Each expansion stage comprises one or moreexpanders, which reduce the pressure of the liquefied stream to therebyevaporate or flash a portion thereof. Examples of suitable expanders caninclude, but are not limited to, Joule-Thompson valves, venturi nozzles,and turboexpanders. Expansion section 12 can employ any number ofexpansion stages and one or more expansion stages may be integrated withone or more cooling stages of third refrigerant chiller 24. In oneembodiment of the present invention, expansion section 12 can reduce thepressure of the LNG-bearing stream in conduit 105 by about 75 to about450 psi, about 125 to about 300 psi, or 150 to 225 psi.

Each expansion stage may additionally employ one or more vapor-liquidseparators operable to separate the vapor phase (i.e., the flash gasstream) from the cooled liquid stream. As previously discussed, thirdrefrigeration cycle 15 can comprise an open-loop refrigeration cycle,closed-loop refrigeration cycle, or any combination thereof. When thirdrefrigeration cycle 15 comprises a closed-loop refrigeration cycle, theflash gas stream can be used as fuel within the facility or routeddownstream for storage, further processing, and/or disposal. When thirdrefrigeration cycle 15 comprises an open-loop refrigeration cycle, atleast a portion of the flash gas stream exiting expansion section 12 canbe used as a refrigerant to cool at least a portion of the natural gasstream in conduit 104. Generally, when third refrigerant cycle 15comprises an open-loop cycle, the third refrigerant can comprise atleast 50 weight percent, at least about 75 weight percent, or at least90 weight percent of flash gas from expansion section 12, based on thetotal weight of the stream. As illustrated in FIG. 1, the flash gasexiting expansion section 12 via conduit 106 can enter third refrigerantchiller 24, wherein the stream can cool at least a portion of thenatural gas stream entering third refrigerant chiller 24 via conduit104. The resulting warmed refrigerant stream can then exit thirdrefrigerant chiller 24 via conduit 108 and can thereafter be routed toan inlet port of third refrigerant compressor 22.

As shown in FIG. 1, third refrigerant compressor 22 discharges a streamof compressed third refrigerant, which is thereafter cooled in cooler23. The cooled refrigerant stream can then be split into two portions.The first portion in conduit 109 a can comprise the domestic gas productstream and can subsequently be routed to a location external to the LNGfacility depicted in FIG. 1. The second portion of cooled refrigerant inconduit 109 b can combine with the natural gas stream in conduit 104prior to re-entering third refrigerant chiller 24, as previouslydiscussed.

As shown in FIG. 1, the liquid stream exiting expansion section 12 viaconduit 107 comprises LNG. In one embodiment, the LNG in conduit 107 canhave a temperature in the range of from about −200 to about −300° F.,about −225 to about −275° F., or −240 to −260° F. and a pressure in therange of from about 0 to about 40 psia, about 5 to about 25 psia, or 10to 20 psia. The LNG in conduit 107 can subsequently be routed to storageand/or shipped to another location via pipeline, ocean-going vessel,truck, or any other suitable transportation means. In one embodiment, atleast a portion of the LNG can be subsequently vaporized for uses inapplications requiring vapor-phase natural gas.

In addition to producing LNG in conduit 107, the LNG facility depictedin FIG. 1 can also produce a domestic gas product in conduit 109 a. Asshown in FIG. 1, the domestic gas product can be withdrawn from anintermediate stream within the LNG facility, typically at a locationdownstream of heavies removal zone 95. Because the domestic gas streamcan be withdrawn downstream of heavies removal zone 95, the domestic gasproduct can have a concentration of C₆+ material that is less than about1 weight percent, less than about 0.5 weight percent, less than about0.1 weight percent, or less than 0.01 weight percent, based on the totalweight of the domestic gas stream. As a result, the domestic gas productwithdrawn from the LNG facility of FIG. 1 via conduit 109 a can complywith most or all of the local natural gas pipeline productspecifications, including, for example, hydrocarbon dew point, withlittle or no additional processing.

In one embodiment shown in FIG. 1, the domestic gas product stream canbe withdrawn from the compressed third refrigerant stream exiting thirdrefrigerant compressor 22 via conduit 109 a. Typically, the pressure ofthe domestic gas stream can be in the range of from about 15 to about100 bar gauge (barg), about 25 to about 90 barg, or 35 to 75 barg. Inorder to produce a domestic gas product having a mass flow rate that isat least about 2 percent, at least about 5 percent, at least about 10percent, or at least 25 percent of the mass flow rate of the totalcompressed third refrigerant stream exiting third refrigerant compressor22, the LNG facility of FIG. 1 can process additional natural gas feed.By processing additional feed gas, additional refrigeration duty can berecovered in the third refrigeration cycle, which can ultimately resultin incremental LNG and/or NGL production. In addition, when the domesticgas product is withdrawn from an open-loop cycle, as illustrated in FIG.1, producing a domestic gas stream can help control the concentration oflight contaminants (e.g., nitrogen) in the refrigeration loop, therebyallowing the LNG facility increased processing flexibility. Further,because of the relatively low concentration of heavies and othercontaminants in the domestic gas product in conduit 109 a, at least aportion of the domestic gas product can subsequently be blended with anunprocessed or off-spec domestic gas stream from another source (notshown) in order to produce a saleable domestic gas product. Optionally,one or more fuel gas streams (not shown) for use within the LNG facilitycan be withdrawn from the domestic gas stream and/or the compressedrefrigerant stream in conduits 109 a, 109 b. Typically, at least aportion of the fuel gas stream can be used to power one or more gasturbine used to drive at least one refrigerant compressor.

FIG. 2 presents one embodiment of a specific configuration of the LNGfacility shown in FIG. 1. While “propane,” “ethylene,” and “methane” areused to refer to respective first, second, and third refrigerants, itshould be understood that the embodiment illustrated in FIG. 2 anddescribed herein can apply to any combination of suitable refrigerants.The LNG facility depicted in FIG. 2 generally comprises a propanerefrigeration cycle 30, an ethylene refrigeration cycle 50, a methanerefrigeration cycle 70 with an expansion section 80, and a heaviesremoval zone 95. To facilitate an understanding of FIG. 2, the followingnumeric nomenclature was employed. Items numbered 31 through 49 areprocess vessels and equipment directly associated with propanerefrigeration cycle 30, and items numbered 51 through 69 are processvessels and equipment related to ethylene refrigeration cycle 50. Itemsnumbered 71 through 94 correspond to process vessels and equipmentassociated with methane refrigeration cycle 70 and/or expansion section80. Items numbered 96 through 99 are process vessels and equipmentassociated with heavies removal zone 95. Items numbered 100 through 199correspond to flow lines or conduits that contain predominantly methanestreams. Items numbered 200 through 299 correspond to flow lines orconduits which contain predominantly ethylene streams. Items numbered300 through 399 correspond to flow lines or conduits that containpredominantly propane streams.

Referring to FIG. 2, the main components of propane refrigeration cycle30 include a propane compressor 31, a propane cooler 32, a high-stagepropane chiller 33, an intermediate stage propane chiller 34, and alow-stage propane chiller 35. The main components of ethylenerefrigeration cycle 50 include an ethylene compressor 51, an ethylenecooler 52, a high-stage ethylene chiller 53, an intermediate-stageethylene chiller 54, a low-stage ethylene chiller/condenser 55, and anethylene economizer 56. The main components of methane refrigerationcycle 70 include a methane compressor 71, a methane cooler 72, a mainmethane economizer 73, and a secondary methane economizer 74. The maincomponents of expansion section 80 include a high-stage methane expander81, a high-stage methane flash drum 82, an intermediate-stage methaneexpander 83, an intermediate-stage methane flash drum 84, a low-stagemethane expander 85, and a low-stage methane flash drum 86. The LNGfacility of FIG. 2 also includes heavies removal zone 95 downstream ofintermediate stage ethylene chiller 54 for removing heavy hydrocarboncomponents from the processed natural gas and recovering the resultingnatural gas liquids. The heavies removal zone 95 of FIG. 2 is shown asgenerally comprising a first distillation column 96 and a seconddistillation column 97.

The operation of the LNG facility illustrated in FIG. 2 will now bedescribed in more detail, beginning with propane refrigeration cycle 30.Propane is compressed in multi-stage (e.g., three-stage) propanecompressor 31 driven by, for example, a gas turbine driver 31 a. Thethree stages of compression preferably exist in a single unit, althougheach stage of compression may be a separate unit and the unitsmechanically coupled to be driven by a single driver. Upon compression,the propane is passed through conduit 300 to propane cooler 32, whereinit is cooled and liquefied via indirect heat exchange with an externalfluid (e.g., air or water). A representative temperature and pressure ofthe liquefied propane refrigerant exiting cooler 32 is about 100° F. andabout 190 psia. The stream from propane cooler 32 can then be passedthrough conduit 302 to a pressure reduction means, illustrated asexpansion valve 36, wherein the pressure of the liquefied propane isreduced, thereby evaporating or flashing a portion thereof. Theresulting two-phase stream then flows via conduit 304 into high-stagepropane chiller 33. High stage propane chiller 33 uses indirect heatexchange means 37, 38, and 39 to cool respectively, the incoming gasstreams, including a yet-to-be-discussed methane refrigerant stream inconduit 112, a natural gas feed stream in conduit 110, and ayet-to-be-discussed ethylene refrigerant stream in conduit 202 viaindirect heat exchange with the vaporizing refrigerant. The cooledmethane refrigerant stream exits high-stage propane chiller 33 viaconduit 130 and can subsequently be routed to the inlet of main methaneeconomizer 73, which will be discussed in greater detail in a subsequentsection.

The cooled natural gas stream from high-stage propane chiller 33 (alsoreferred to herein as the “methane-rich stream”) flows via conduit 114to a separation vessel 40, wherein the gaseous and liquid phases areseparated. The liquid phase, which can be rich in propane and heaviercomponents (C₃+), is removed via conduit 303. The predominately vaporphase exits separator 40 via conduit 116 and can then enterintermediate-stage propane chiller 34, wherein the stream is cooled inindirect heat exchange means 41 via indirect heat exchange with ayet-to-be-discussed propane refrigerant stream. The resulting two-phasemethane-rich stream in conduit 118 can then be routed to low-stagepropane chiller 35, wherein the stream can be further cooled viaindirect heat exchange means 42. The resultant predominantly methanestream can then exit low-stage propane chiller 34 via conduit 120.Subsequently, the cooled methane-rich stream in conduit 120 can berouted to high-stage ethylene chiller 53, which will be discussed inmore detail shortly.

The vaporized propane refrigerant exiting high-stage propane chiller 33is returned to the high-stage inlet port of propane compressor 31 viaconduit 306. The residual liquid propane refrigerant in high-stagepropane chiller 33 can be passed via conduit 308 through a pressurereduction means, illustrated here as expansion valve 43, whereupon aportion of the liquefied refrigerant is flashed or vaporized. Theresulting cooled, two-phase refrigerant stream can then enterintermediate-stage propane chiller 34 via conduit 310, thereby providingcoolant for the natural gas stream and yet-to-be-discussed ethylenerefrigerant stream entering intermediate-stage propane chiller 34. Thevaporized propane refrigerant exits intermediate-stage propane chiller34 via conduit 312 and can then enter the intermediate-stage inlet portof propane compressor 31. The remaining liquefied propane refrigerantexits intermediate-stage propane chiller 34 via conduit 314 and ispassed through a pressure-reduction means, illustrated here as expansionvalve 44, whereupon the pressure of the stream is reduced to therebyflash or vaporize a portion thereof. The resulting vapor-liquidrefrigerant stream then enters low-stage propane chiller 35 via conduit316 and cools the methane-rich and yet-to-be-discussed ethylenerefrigerant streams entering low-stage propane chiller 35 via conduits118 and 206, respectively. The vaporized propane refrigerant stream thenexits low-stage propane chiller 35 and is routed via conduit 318 to thelow-stage inlet port of propane compressor 31, wherein the stream iscompressed and recycled as previously described.

As shown in FIG. 2, a stream of ethylene refrigerant in conduit 202enters high-stage propane chiller, wherein the ethylene stream is cooledvia indirect heat exchange means 39. The resulting cooled stream inconduit 204 then exits high-stage propane chiller 33, whereafter the atleast partially condensed stream enters intermediate-stage propanechiller 34. Upon entering intermediate-stage propane chiller 34, theethylene refrigerant stream can be further cooled via indirect heatexchange means 45. The resulting two-phase ethylene stream can then exitintermediate-stage propane chiller 34 prior to entering low-stagepropane chiller 35 via conduit 206. In low-stage propane chiller 35, theethylene refrigerant stream can be at least partially condensed, orcondensed in its entirety, via indirect heat exchange means 46. Theresulting stream exits low-stage propane chiller 35 via conduit 208 andcan subsequently be routed to a separation vessel 47, wherein the vaporportion of the stream, if present, can be removed via conduit 210. Theliquefied ethylene refrigerant stream exiting separator 47 via conduit212 can have a representative temperature and pressure of about −24° F.and about 285 psia.

Turning now to ethylene refrigeration cycle 50 in FIG. 2, the liquefiedethylene refrigerant stream in conduit 212 can enter ethylene economizer56, wherein the stream can be further cooled by an indirect heatexchange means 57. The sub-cooled liquid ethylene stream in conduit 214can then be routed through a pressure reduction means, illustrated hereas expansion valve 58, whereupon the pressure of the stream is reducedto thereby flash or vaporize a portion thereof. The cooled, two-phasestream in conduit 215 can then enter high-stage ethylene chiller 53,wherein at least a portion of the ethylene refrigerant stream canvaporize to thereby cool the methane-rich stream entering an indirectheat exchange means 59 of high-stage ethylene chiller 53 via conduit120. The vaporized and remaining liquefied refrigerant exit high-stageethylene chiller 53 via respective conduits 216 and 220. The vaporizedethylene refrigerant in conduit 216 can re-enter ethylene economizer 56,wherein the stream can be warmed via an indirect heat exchange means 60prior to entering the high-stage inlet port of ethylene compressor 51via conduit 218, as shown in FIG. 2.

The remaining liquefied refrigerant in conduit 220 can re-enter ethyleneeconomizer 56, wherein the stream can be further sub-cooled by anindirect heat exchange means 61. The resulting cooled refrigerant streamexits ethylene economizer 56 via conduit 222 and can subsequently berouted to a pressure reduction means, illustrated here as expansionvalve 62, whereupon the pressure of the stream is reduced to therebyvaporize or flash a portion thereof. The resulting, cooled two-phasestream in conduit 224 enters intermediate-stage ethylene chiller 54,wherein the refrigerant stream can cool the natural gas stream inconduit 122 entering intermediate-stage ethylene chiller 54 via anindirect heat exchange means 63. As shown in FIG. 2, the resultingcooled methane-rich stream exiting intermediate stage ethylene chiller54 can then be routed to heavies removal zone 95 via conduit 124.Heavies removal zone 95 will be discussed in detail in a subsequentsection.

The vaporized ethylene refrigerant exits intermediate-stage ethylenechiller 54 via conduit 226, whereafter the stream can combine with ayet-to-be-discussed ethylene vapor stream in conduit 238. The combinedstream in conduit 239 can then enter ethylene economizer 56, wherein thestream is warmed in an indirect heat exchange means 64 prior to beingfed into the low-stage inlet port of ethylene compressor 51 via conduit230. Ethylene compressor 51 can be driven by, for example, a gas turbinedriver 51 a. Ethylene compressor 51 comprises at least one stage ofcompression, and, when multiple stages are employed, the stages canexist in a single unit or can be separate units mechanically coupled toa common driver. Generally, when ethylene compressor 71 comprises two ormore compression stages, one or more intercoolers (not shown) can beprovided between subsequent compression stages. As shown in FIG. 2, astream of compressed ethylene refrigerant in conduit 236 cansubsequently be routed to ethylene cooler 52, wherein the ethylenestream can be cooled via indirect heat exchange with an external fluid(e.g., water or air). The resulting, at least partially condensedethylene stream can then be introduced via conduit 202 into high-stagepropylene chiller 33 for additional cooling as previously described.

The remaining liquefied ethylene refrigerant exits intermediate-stageethylene chiller 54 via conduit 228 prior to entering low-stage ethylenechiller/condenser 55, wherein the refrigerant can cool the methane-richstream entering low-stage ethylene chiller/condenser via conduit 128 inan indirect heat exchange means 65. In one embodiment shown in FIG. 2,the stream in conduit 128 results from the combination of aheavies-depleted (i.e., light hydrocarbon rich) stream exiting heaviesremoval zone 95 via conduit 126 and a yet-to-be-discussed methanerefrigerant stream in conduit 168. As shown in FIG. 2, the vaporizedethylene refrigerant can then exit low-stage ethylene chiller/condenser55 via conduit 238 prior to combining with the vaporized ethyleneexiting intermediate-stage ethylene chiller 54 via conduit 226 andentering the low-stage inlet port of ethylene compressor 51, aspreviously discussed.

The cooled natural gas stream exiting low-stage ethylenechiller/condenser in conduit 132 can also be referred to as the“pressurized LNG-bearing stream.” As shown in FIG. 2, the pressurizedLNG-bearing stream exits low-stage ethylene chiller/condenser 55 viaconduit 132 prior to entering main methane economizer 73. In mainmethane economizer 73, the methane-rich stream can be cooled in anindirect heat exchange means 75 via indirect heat exchange with one ormore yet-to-be discussed methane refrigerant streams. The cooled,pressurized LNG-bearing stream exits main methane economizer 73 and canthen be routed via conduit 134 into expansion section 80 of methanerefrigeration cycle 70. In expansion section 80, the cooledpredominantly methane stream passes through high-stage methane expander81, whereupon the pressure of the stream is reduced to thereby vaporizeor flash a portion thereof. The resulting two-phase methane-rich streamin conduit 136 can then enter high-stage methane flash drum 82,whereupon the vapor and liquid portions can be separated. The vaporportion exiting high-stage methane flash drum 82 (i.e., the high-stageflash gas) via conduit 143 can then enter main methane economizer 73,wherein the stream is heated via indirect heat exchange means 76. Theresulting warmed vapor stream exits main methane economizer 73 viaconduit 138 and subsequently combines with a yet-to-be-discussed vaporstream exiting heavies removal zone 95 in conduit 140. The combinedstream in conduit 141 can then be routed to the high-stage inlet port ofmethane compressor 71, as shown in FIG. 2.

The liquid phase exiting high-stage methane flash drum 82 via conduit142 can enter secondary methane economizer 74, wherein the methanestream can be cooled via indirect heat exchange means 92. The resultingcooled stream in conduit 144 can then be routed to a second expansionstage, illustrated here as intermediate-stage expander 83, wherein thepressure of the stream can be reduced to thereby evaporate or flash aportion thereof. The resulting two-phase methane-rich stream in conduit146 can then enter intermediate-stage methane flash drum 84, wherein theliquid and vapor portions of the stream can be separated and can exitthe intermediate-stage flash drum via respective conduits 148 and 150.The vapor portion (i.e., the intermediate-stage flash gas) in conduit150 can re-enter secondary methane economizer 74, wherein the stream canbe heated via an indirect heat exchange means 87. The warmed stream canthen be routed via conduit 152 to main methane economizer 73, whereinthe stream can be further warmed via an indirect heat exchange means 77prior to entering the intermediate-stage inlet port of methanecompressor 71 via conduit 154.

The liquid stream exiting intermediate-stage methane flash drum 84 viaconduit 148 can then pass through a low-stage expander 85, whereupon thepressure of the liquefied methane-rich stream can be further reduced tothereby vaporize or flash a portion thereof. The resulting cooled,two-phase stream in conduit 156 can then enter low-stage methane flashdrum 86, wherein the vapor and liquid phases can be separated. Theliquid stream exiting low-stage methane flash drum 86 can comprise theliquefied natural gas (LNG) product. The LNG product, which is at aboutatmospheric pressure, can be routed via conduit 158 downstream forsubsequent storage, transportation, and/or use.

The vapor stream exiting low-stage methane flash drum 86 (i.e., thelow-stage methane flash gas) in conduit 160 can be routed to secondarymethane economizer 74, wherein the stream can be warmed via an indirectheat exchange means 89. The resulting stream can exit secondary methaneeconomizer 74 via conduit 162, whereafter the stream can be routed tomain methane economizer 73 to be further heated via indirect heatexchange means 78. The warmed methane vapor stream can then exit mainmethane economizer 73 via conduit 164, whereafter the stream can besplit into two portions. The first portion in conduit 164 can enter thelow-stage inlet port of methane compressor 71, which will be discussedin detail shortly. The second portion in conduit 164 a can be routed toan inlet port of a sales gas compressor 91. The compressed gas productexiting sales gas compressor 91 via conduit 172 e can then cooled (notshown) and routed to a location external to the LNG facility for use asa domestic gas product. Optionally, as shown in FIG. 2, at least aportion of the compressed gas stream in conduit 172 e can be routed viaconduit 160 b to recombine with the warmed refrigerant stream in conduit164.

As previously discussed, the warmed methane refrigerant stream inconduit 164 can enter the low-stage inlet port of methane compressor 71.Methane compressor 71 can be driven by, for example, a gas turbinedriver 71 a. Methane compressor 71 comprises at least one stage ofcompression, and, when multiple stages are employed, the stages canexist in a single unit or can be separate units mechanically coupled toa common driver. Generally, when methane compressor 71 comprises two ormore compression stages, one or more intercoolers (not shown) can beprovided between subsequent compression stages.

As shown in FIG. 2, the compressed methane refrigerant stream exitingmethane compressor 71 can be discharged into conduit 166, whereafter thestream can be cooled via indirect heat exchange with an external fluid(e.g., air or water) in methane cooler 72. In one embodiment, the cooledcompressed refrigerant stream can then be split into a compressedrefrigerant fraction in conduit 112 and a domestic gas fraction inconduit 172 a. Optionally, a fuel gas stream can be withdrawn from thedomestic gas fraction via conduit 174 a and/or from the compressedrefrigerant fraction via conduit 176 a. The domestic gas fraction inconduit 172 a can subsequently be routed to a location outside the LNGfacility, whereafter the domestic gas stream can optionally be combinedwith another gas stream (e.g., a portion of the feed natural gas) priorto being transported and sold to subsequent users. The fuel gas stream,if present, can be routed to one or more fuel gas consumers (e.g., gasturbine drivers 31 a, 51 a, and 71 a of respective propane, ethylene,and methane compressors 31, 51, 71) within the LNG facility. In anotherembodiment, a domestic gas fraction can be withdrawn from the streamsexiting the discharge of the low-stage, intermediate-stage, and/orhigh-stage of methane compressor 71, as indicated in FIG. 1 byrespective lines 172 b, 172 c, 172 d. In addition, optional fuel gasstreams 174 b-d can be withdrawn from the domestic gas fractions incorresponding conduits 172 b-d or from the remaining compressedrefrigerant fractions exiting the low, intermediate, and high stages ofmethane compressor 71 (not shown). As illustrated in FIG. 2, thecompressed refrigerant fraction in conduit 112 can be further cooled inpropane refrigeration cycle 30, as described in detail previously.

Upon being cooled in propane refrigeration cycle 30, the compressedmethane refrigerant fraction can be discharged into conduit 130 andsubsequently routed to main methane economizer 73, wherein the streamcan be further cooled via indirect heat exchange means 79. The resultingsub-cooled stream exits main methane economizer 73 via conduit 168 andcan then combined with the heavies-depleted stream exiting heaviesremoval zone 95 via conduit 126, as previously discussed.

Turning now to heavies removal zone 95, the cooled, at least partiallycondensed effluent exiting intermediate-stage ethylene chiller 54 viaconduit 124 can be routed into the inlet of first distillation column96, as shown in FIG. 2. A predominantly methane vapor overhead productstream can exit an upper outlet of first distillation column 96 viaconduit 126. As discussed previously, the stream in conduit 126 cansubsequently combine with the methane refrigerant stream in conduit 168prior to entering low-stage ethylene chiller/condenser 55 via conduit128. Referring back to heavies removal zone 95, a heavies-rich bottomsliquid product stream exiting a lower outlet of first distillationcolumn 96 via conduit 170 can then be routed to an inlet of seconddistillation column 97. An overhead vapor product stream can exit anupper outlet of second distillation column 97 via conduit 140 prior tobeing combined with the warmed methane refrigerant stream in conduit138, as discussed in detail previously. The bottoms liquid productexiting a lower outlet of second distillation column 97 can comprise thenatural gas liquids (NGL) product. The NGL product, which can comprise asignificant concentration of butane and heavier hydrocarbons, such asbenzene, cyclohexane, and other aromatics, can be routed to furtherstorage, processing, and/or use via conduit 171.

In one embodiment of the present invention, the LNG production systemsillustrated in FIGS. 1 and 2 are simulated on a computer usingconventional process simulation software in order to produce simulationresults. In one embodiment, the simulation results can be in the form ofa computer print out. In another embodiment, the simulation results canbe displayed on a screen, monitor, or other viewing device. In yetanother embodiment, the simulation results may be electronic signalsdirectly communicated into the LNG system for direct control and/oroptimization of the system.

The simulation results can then be used to manipulate the LNG system. Inone embodiment, the simulation results can be used to design a new LNGfacility and/or revamp or expand an existing LNG facility. In anotherembodiment, the simulation results can be used to optimize the LNGfacility according to one or more operating parameters. In a furtherembodiment, the computer simulation can directly control the operationof the LNG facility by, for example, manipulating control valve output.Examples of suitable software for producing the simulation resultsinclude HYSYS™ or Aspen Plus® from Aspen Technology, Inc., and PRO/II®from Simulation Sciences Inc.

Numerical Ranges

The present description uses numerical ranges to quantify certainparameters relating to the invention. It should be understood that whennumerical ranges are provided, such ranges are to be construed asproviding literal support for claim limitations that only recite thelower value of the range as well as claims limitation that only recitethe upper value of the range. For example, a disclosed numerical rangeof 10 to 100 provides literal support for a claim reciting “greater than10” (with no upper bounds) and a claim reciting “less than 100” (with nolower bounds).

Definitions

As used herein, the terms “a,” “an,” “the,” and “said” means one ormore.

As used herein, the term “and/or,” when used in a list of two or moreitems, means that any one of the listed items can be employed by itself,or any combination of two or more of the listed items can be employed.For example, if a composition is described as containing components A,B, and/or C, the composition can contain A alone; B alone; C alone; Aand B in combination; A and C in combination; B and C in combination; orA, B, and C in combination.

As used herein, the term “cascade-type refrigeration process” refers toa refrigeration process that employs a plurality of refrigerationcycles, each employing a different pure component refrigerant tosuccessively cool natural gas.

As used herein, the term “closed-loop refrigeration cycle” refers to arefrigeration cycle wherein substantially no refrigerant enters or exitsthe cycle during normal operation.

As used herein, the terms “comprising,” “comprises,” and “comprise” areopen-ended transition terms used to transition from a subject recitedbefore the term to one or elements recited after the term, where theelement or elements listed after the transition term are not necessarilythe only elements that make up of the subject.

As used herein, the terms “containing,” “contains,” and “contain” havethe same open-ended meaning as “comprising,” “comprises,” and“comprise,” provided below.

As used herein, the term “domestic gas product” refers to any gaseous,predominantly methane stream originating from within an LNG facilitythat is routed to a location external to the LNG facility prior to saleand/or use.

As used herein, the terms “economizer” or “economizing heat exchanger”refer to a configuration utilizing a plurality of heat exchangersemploying indirect heat exchange means to efficiently transfer heatbetween process streams.

As used herein, the terms “having,” “has,” and “have” have the sameopen-ended meaning as “comprising,” “comprises,” and “comprise,”provided above.

As used herein, the terms “heavy hydrocarbon” and “heavies” refer to anyhydrocarbon component having a molecular weight greater than methane.

As used herein, the terms “including,” “includes,” and “include” havethe same open-ended meaning as “comprising,” “comprises,” and“comprise,” provided above.

As used herein, the term “mid-range standard boiling point” refers tothe temperature at which half of the weight of a mixture of physicalcomponents has been vaporized (i.e., boiled off) at standard pressure.

As used herein, the term “mixed refrigerant” refers to a refrigerantcontaining a plurality of different components, where no singlecomponent makes up more than 75 mole percent of the refrigerant.

As used herein, the term “natural gas” means a stream containing atleast 85 mole percent methane, with the balance being ethane, higherhydrocarbons, nitrogen, carbon dioxide, and/or a minor amount of othercontaminants such as mercury, hydrogen sulfide, and mercaptan.

As used herein, the terms “natural gas liquids” or “NGL” refer tomixtures of hydrocarbons whose components are, for example, typicallyheavier than ethane. Some examples of hydrocarbon components of NGLstreams include propane, butane, and pentane isomers, benzene, toluene,and other aromatic compounds.

As used herein, the term “open-loop refrigeration cycle” refers to arefrigeration cycle wherein at least a portion of the refrigerantemployed during normal operation originates from an external source.

As used herein, the terms “predominantly,” “primarily,” “principally,”and “in major portion,” when used to describe the presence of aparticular component of a fluid stream, means that the fluid streamcomprises at least 50 mole percent of the stated component. For example,a “predominantly” methane stream, a “primarily” methane stream, a stream“principally” comprised of methane, or a stream comprised “in majorportion” of methane each denote a stream comprising at least 50 molepercent methane.

As used herein, the term “pure component refrigerant” means arefrigerant that is not a mixed refrigerant.

As used herein, the terms “upstream” and “downstream” refer to therelative positions of various components of a natural gas liquefactionfacility along the main flow path of natural gas through the plant.

Claims not Limited to Disclosed Embodiments

The preferred forms of the invention described above are to be used asillustration only, and should not be used in a limiting sense tointerpret the scope of the present invention. Modifications to theexemplary embodiments, set forth above, could be readily made by thoseskilled in the art without departing from the spirit of the presentinvention.

The inventors hereby state their intent to rely on the Doctrine ofEquivalents to determine and assess the reasonably fair scope of thepresent invention as pertains to any apparatus or process not materiallydeparting from but outside the literal scope of the invention as setforth in the following claims.

1. A process for liquefying a natural gas stream in an LNG facility,said process comprising: (a) cooling at least a portion of said naturalgas stream in a first refrigeration cycle via indirect heat exchangewith a predominantly methane refrigerant; (b) flashing at least aportion of the cooled natural gas stream to thereby provide apredominantly liquid product stream and a predominantly vaporrefrigerant stream; (c) compressing at least a portion of saidpredominantly vapor refrigerant stream to thereby provide a compressedrefrigerant stream; and (d) separating at least a portion of saidcompressed refrigerant stream into a domestic gas fraction and acompressed refrigerant fraction.
 2. The process of claim 1, furthercomprising, prior to step (a), cooling at least a portion of saidnatural gas stream in an upstream refrigeration cycle via indirect heatexchange with an upstream refrigerant.
 3. The process of claim 2,further comprising cooling at least a portion of said compressedrefrigerant fraction in said upstream refrigeration cycle.
 4. Theprocess of claim 3, wherein said upstream refrigeration cycle comprisesa closed-loop refrigeration cycle.
 5. The process of claim 3, whereinsaid upstream refrigerant comprises a pure component refrigerant.
 6. Theprocess of claim 3, wherein said upstream refrigerant comprises propaneand/or ethylene.
 7. The process of claim 1, further comprising, prior tostep (d) and subsequent to step (c), cooling at least a portion of saidcompressed refrigerant stream.
 8. The process of claim 1, wherein themass flow rate of said domestic gas fraction is at least about 2 percentof the mass flow rate of said compressed refrigerant stream.
 9. Theprocess of claim 1, further comprising, optionally, withdrawing a fuelgas stream from said compressed refrigerant fraction and/or saiddomestic gas fraction.
 10. The process of claim 1, wherein said vaporrefrigerant stream provides at least a portion of said cooling of step(a).
 11. The process of claim 1, further comprising producing liquefiednatural gas (LNG) that comprises at least a portion of saidpredominantly liquid product stream.
 12. The process of claim 11,wherein the produced LNG is at about atmospheric pressure.
 13. Theprocess of claim 1, further comprising routing said domestic gasfraction to a location outside of said LNG facility, wherein saidrouting does not require the use of a compressor other than thecompressor or compressors used to carry out step (d).
 14. The process ofclaim 1, wherein said domestic gas fraction has a pressure in the rangeof from about 35 to about 75 barg.
 15. The process of claim 1, furthercomprising vaporizing LNG produced via steps (a)-(d).
 16. A computersimulation process comprising utilizing a computer to simulate theprocess of claim
 1. 17. A process for liquefying a natural gas stream inan LNG facility, said process comprising: (a) cooling at least a portionof said natural gas stream in an upstream refrigeration cycle of saidLNG facility via indirect heat exchange with an upstream refrigerant tothereby provide a cooled natural gas stream; (b) further cooling atleast a portion of said cooled natural gas stream via indirect heatexchange with a predominantly methane refrigerant stream in a methanerefrigeration cycle to thereby produce a further cooled natural gasstream and a warmed refrigerant stream; (c) separating at least aportion of said warmed refrigerant stream into a domestic gas fractionand a refrigerant fraction; and (d) cooling at least a portion of saidrefrigerant fraction in said upstream refrigeration cycle via indirectheat exchange with said upstream refrigerant.
 18. The process of claim17, further comprising, prior to step (c), flashing at least a portionof said further cooled natural gas stream to thereby produce a vaporrefrigerant stream and a liquid product stream.
 19. The process of claim17, further comprising, prior to step (c), compressing at least aportion of said warmed refrigerant stream, wherein said domestic gasfraction comprises at least a portion of the compressed warmedrefrigerant.
 20. The process of claim 19, further comprising, prior tostep (c), cooling at least a portion of said compressed warmedrefrigerant to thereby produce a compressed cooled refrigerant, whereinsaid domestic gas fraction comprises at least a portion of saidcompressed cooled refrigerant.
 21. The process of claim 17, wherein themass flow rate of said domestic gas fraction is at least about 2 percentof the mass flow rate of said warmed refrigerant stream.
 22. The processof claim 17, further comprising, optionally, withdrawing a fuel gasstream from said refrigerant fraction and/or said domestic gas fraction.23. The process of claim 17, wherein said upstream refrigeration cyclecomprises a closed-loop refrigeration cycle.
 24. The process of claim17, wherein said methane refrigeration cycle comprises an open-looprefrigeration cycle.
 25. The process of claim 17, wherein said upstreamrefrigerant comprises ethylene and/or ethane.
 26. The process of claim17, wherein said upstream refrigerant comprises propylene and/orpropane.
 27. The process of claim 17, wherein said domestic gas fractionhas a pressure in the range of from about 35 to about 75 barg.
 28. Theprocess of claim 17, further comprising, vaporizing LNG produced viasteps (a)-(d).
 29. A computer simulation process comprising utilizing acomputer to simulate the process of claim
 17. 30. A process forliquefying a natural gas stream in an LNG facility, said processcomprising: (a) cooling at least a portion of said natural gas stream ina first refrigeration cycle via indirect heat exchange with a firstrefrigerant to thereby produce a cooled predominantly methane stream;(b) separating at least a portion of said cooled predominantly methanestream in a distillation column to thereby provide a heavies-rich streamand a heavies-depleted stream; (c) subjecting at least a portion of saidheavies-depleted stream to expansion cooling to thereby produce LNGhaving a pressure in the range of from about 0 to about 40 psia; and (d)prior to at least a portion of said expansion cooling of step (c),withdrawing a domestic gas fraction from said heavies-depleted stream.31. The process of claim 30, wherein said expansion cooling includesflashing at least a portion of said heavies-depleted stream to therebyproduce a predominantly vapor stream and a predominantly liquid stream,wherein said domestic gas fraction comprises at least a portion of saidpredominantly vapor stream, wherein said LNG comprises at least aportion of said predominantly liquid stream.
 32. The process of claim31, further comprising using at least a portion of said predominantlyvapor stream to cool at least a portion of said heavies-depleted streamvia indirect heat exchange in a second refrigeration cycle to therebyproduce a warmed predominantly vapor stream, wherein said domestic gasproduct comprises at least a portion of said warmed predominantly vaporstream.
 33. The process of claim 32, further comprising compressing atleast a portion of said warmed predominantly vapor stream to therebyproduce a compressed warmed predominantly vapor stream, wherein saiddomestic gas product comprises at least a portion of said compressedwarmed predominantly vapor stream.
 34. The process of claim 30, furthercomprising, prior to step (c), cooling at least a portion of saidheavies-depleted stream in a second refrigeration cycle via indirectheat exchange with a second refrigerant.
 35. The process of claim 34,wherein said second refrigeration cycle comprises an open-loop methanerefrigeration cycle.
 36. An LNG facility for liquefying a natural gasstream, said LNG facility comprising: an open-loop refrigeration cycleoperable to cool at least a portion of said natural gas stream viaindirect heat exchange with a first refrigerant, wherein said open-looprefrigeration cycle comprises— a first heat exchanger defining a firstcooling pass and a first refrigerant pass, wherein said first heatexchanger is operable to cool at least a portion of said natural gasstream in said first cooling pass via indirect heat exchange with saidfirst refrigerant in said first refrigerant pass; an expander definingan expander inlet and an expander outlet, wherein said expander inlet iscoupled in fluid communication with said first cooling pass; avapor-liquid separator defining a separator inlet, a lower liquidoutlet, and an upper vapor outlet, wherein said separator inlet iscoupled in fluid flow communication with said expander outlet, whereinsaid upper vapor outlet is coupled in fluid flow communication with saidfirst refrigerant pass; a first refrigerant compressor defining an inletport and an outlet port, wherein said inlet port is coupled in fluidflow communication with said first refrigerant pass; a compressedrefrigerant conduit for routing fluid flow from said outlet port of saidcompressor to a location within said LNG facility; and a domestic gasconduit for routing fluid flow from said outlet port of said compressorand/or said compressed refrigerant conduit to a location outside saidLNG facility.
 37. The facility of claim 36, further comprising aclosed-loop refrigeration cycle located upstream of said open-looprefrigeration cycle, wherein said closed-loop refrigeration cycle isoperable to cool at least a portion of said natural gas stream viaindirect heat exchange with a second refrigerant.
 38. The facility ofclaim 36, wherein said location within said LNG facility is in saidclosed-loop refrigeration cycle.
 39. The facility of claim 37, whereinsaid closed-loop refrigeration cycle is operable to cool the fluidrouted thereto by said compressed refrigerant conduit via indirect heatexchange with said second refrigerant.
 40. The facility of claim 37,wherein said closed-loop refrigeration cycle comprises a second heatexchanger defining a second cooling pass and a second refrigerant pass,wherein said second heat exchanger is operable to cool at least aportion of said natural gas stream in said second cooling pass viaindirect heat exchange with said second refrigerant in said secondrefrigerant pass.
 41. The facility of claim 40, wherein said second heatexchanger further comprises a third cooling pass, wherein said locationinside said LNG facility is said third cooling pass, wherein said secondheat exchanger is operable to cool fluid routed to said third coolingpass by said compressed refrigerant conduit via indirect heat exchangewith said second refrigerant in said second refrigerant pass.
 42. Thefacility of claim 36, wherein said LNG facility comprises a plurality ofgas turbines each defining a fuel gas inlet, wherein said domestic gasconduit does not route fluid flow to any of said fuel gas inlets. 43.The facility of claim 36, further comprising at least two downstreamexpanders coupled in fluid communication with said expander outlet. 44.The facility of claim 43, further comprising a downstream vapor-liquidseparator coupled between and in fluid flow communication with said atleast two downstream expanders.
 45. The facility of claim 44, whereinsaid first heat exchanger further comprises an additional refrigerantpass coupled in fluid flow communication with said downstreamvapor-liquid separator.
 46. The facility of claim 45, wherein said firstrefrigerant compressor further defines an additional inlet port coupledin fluid flow communication with said additional refrigerant pass. 47.The facility of claim 36, further comprising a third heat exchangerinterposed in said compressed refrigerant conduit and operable to coolfluids flowing through said compressed refrigerant conduit, wherein saiddomestic gas conduit is coupled in fluid flow communication with saidcompressed refrigerant conduit downstream of said third heat exchanger.48. The facility of claim 36, wherein said LNG facility is acascade-type LNG facility.